Method and apparatus for forming corrugated plastic tubing

ABSTRACT

Corrugated plastic tubing is formed by producing a plastic tube in a softened condition, passing the softened tube through a rotatable disc having an axial bore with a helical groove for corrugating the wall of the tubular body and then cooling the tube to set the corrugations formed in the wall of the tube. Means is provided for rotating the die, and means synchronized with the rotating means is also provided for drawing the tubular body through the die and cooling chamber to form a helical corrugation in the tubing wall. A pressure differential is applied in the die across the wall of the tubular body to force the wall of the tube into the die groove. Means is provided for engaging the softened tubular body adjacent the entrance to the die to restrain the tubular body against rotation with the die and prevent the softened tubular body from becoming twisted or kinked before entering the die.

States ate Hetrich [451 Sept. 19, 1972 Inventor:

Arthur Ronald Hetrich, Manheim, Pa.

Assignee: Raybestos-Manhattan, lnc.,

heim, Pa.

Filed: March 17, 1970 Appl. No.:' 20,174

Related US. Application Data Continuation-impart of Ser. No. 749,736,Aug. 2, 1968, abandoned.

[73] Man- US. Cl. .264/92, 18/14 RR, 18/19 TC, 264/99, 264/ 167,264/209, 264/290 R Int. Cl. ..B29c 17/07, B29d 23/18 Field of Search..264/89, 90, 92, 95, 99,- 127, 264/167, 209, 230, 280, 290 R; 18/14 A,14 RR, 19 TC References Cited UNITED STATES PATENTS 2/1967 Pepper 18/14A X 11/1966 Boggs ..l8/l4AX gill "l e C 7 -42 2,423,260 7/ 1947Slaughter 18/14 A X 3,529,047 9/1970 Yoshida et al ..264/90 X 3,296,6611! 1967 De Moustier .l8/l4 A 3,086,242 4/ 1963 Cook et a1 18/14 A XPrimary Examiner-Robert F. White Assistant Examiner-J. H. SilbaughAttomey-Howson and Howson [5 7 ABSTRACT Corrugated plastic tubing isformed by producing a plastic tube in a softened condition, passing thesoftened tube through a rotatable disc having an axial bore with ahelical groove for corrugating the wall of the tubular body and thencooling the tube to set the corrugations formed in thewall of the tube.Means is provided for rotating the die, and means synchronized with therotating means is also provided for drawing the tubular body through thedie and cooling chamber to form a helical corrugation in the tubingwall. A pressure differential is applied in the die across the wall ofthe tubular body to force the wall of the tube into the die groove.Means is provided for engaging the softened tubular body adjacent theentrance to the die to restrain the tubular body against rotation withthe die and prevent the softened tubular body from becoming twisted orkinked before entering the die.

2 Claims, 6 Drawing Figures PATENTED SEP 19 m2 SHEET 1 BF 4 INVENTOR-ARTHUR RONALD HETR ICH A'I'TYS,

PATENTEDSEP 19 I972 SHEET 2 [IF 4 FIG 2.

FIGS.

. INVENTOR ARTHUR RONALD HETR ICH ATT 5 PATENTED E 1 9 F97? sum 3 0F 4ATTYS.

METHOD AND APPARATUS FOR F? i NG CO f UGATED PLASTIC TUBING Thisapplication is a continuation-in-part of my earlier-filed cop'endingapplication, Ser. No. 749,736, filed Aug. 2, 1968, now abandoned.

The present invention relates to a method and apparatus for formingcorrugated plastic tubing, and more particularly, the present inventionrelates to a method and apparatus for forming corrugated plastic tubinghaving a helical corrugation in the tube wall.

The increased use of plastic tubing in various applications in recentyears has generated demands for techniques and machinery to continuouslyproduce large quantities of tubing at relatively low costs. In someapplications, for instance in conduits in aircraft electrical systems,it is desirable for the tubing to be sufficiently flexible to permit itto be bent into tortuous configurations. In addition, the operation ofaircraft at high altitudes and under various G-loading conditionsrequires that the tubing not only possess good dielectric strength, butalso that it possess good mechanical strength by having a minimum ofinternal stresses within the conduit, as would be present in thevicinity of a seam, for example. Tubing formed of fluorinated ethylene,propylene polymers possesses good dielectric strength; however,heretofore apparatus has not been available for forming continuous,seamless lengths of corrugated tubing from such polymers.

For example, there are a number of known processes and apparatus in theprior art for forming corrugated plastic tubing; however, each hascertain disadvantages. There is apparatus in which separate die segmentsare used for molding the wall of a plastic tube. Such apparatus does notproduce a seamless tube and is generally limited to forming relativelyshort lengths of tubing. Although apparatus is available wherebycontinuous lengths of corrugated tubing may be formed using separate diesegments which mold the tubing wall while moving axially along with thetubing; nevertheless, this apparatus is complex and expensive. Similardisadvantages are inherent in conventional blow-molding machinery, and,in addition, conventional blowmolding machinery is not adapted toproduce continuous lengths of corrugated tubing. Furthermore, each ofthe aforementioned tube-forming devices is not readily adapted to bequickly changed to produce tubing of different sizes andcharacteristics, and skilled operators are required to effect thedesired changes.

With the foregoing in mind, it is a primary object of the presentinvention to provide an improved method and apparatus for continuouslyforming seamless, helically corrugated, plastic tubing.

It is another object of the present invention to provide noveltube-forming apparatus which is compact and capable of producingseamless corrugated tubing in a continuous process.

As a further object, the present invention provides tube-formingapparatus which is unique and particula'rly adapted to producecorrugated tubing in which the sizes and characteristics of the tubingmay be changed quickly and with a minimum amount of tools and skill.

More particularly, apparatus of the present invention comprises meansfor producing a plastic tube in a softened condition, a die forreceiving the softened tube having a helically grooved axial boremounted for rotation about its axis, and means for cooling the tube toset the helical groove in the wall thereof. Means is provided forrotating the die, and means is mounted at the entrance of the die forfrictionally engaging the exterior surface of the tubing wall to preventthe tubing from rotating with the die so that the softened tubing willnot be twisted or kinked before entering the rotating die. The softenedtubular body is drawn axially through the die by means synchronized withthe die rotating means, and means is provided for producing a pressuredifferential in the die across the wall of the tubing to urge the tubewall into the groove to thereby form a helical corrugation in the wall.

In addition to the foregoing, other objects, features and advantageswill become apparent from the following description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a side elevational view of one embodiment of tube-formingapparatus embodying the present invention, with portions broken away;

FIG. 2 is an enlarged fragmentary view in longitudinal section of theanti-rotation plate and die arrangement illustrated in FIG. 1;

FIG. 3 is an enlarged fragmentary view in longitudinal section of a diehaving a modified passage arrangement;

FIG. 4 is a side elevational view of another embodiment of apparatus forproducing corrugated tubing from pre-formed tubing which is notmelt-extrudable;

FIG. 5 is an enlarged fragmentary view in longitudinal section of thedie area of the apparatus shown in FIG. 4; and

FIG. 6 is a sectional view taken on line 6-6, FIG. 5.

According to the conventional tube-forming practice, a plastic tube isproduced in a softened condition, and the softened tube then is set orquenched by passing it through a cooling medium. When it is desired toimpart a helical configuration to the wall of the tubing, it has oftenbeen necessary to process the tubing in a separate step with appropriateforming apparatus. Thus, this conventional process is undesirable notonly because it is relatively slow, but also because additional handlingof the tubing is required. Accordingly, the method of the presentinvention obviates these disadvantages by producing seamless, helicallycorrugated plastic tubing in a continuous process. To this end, thesteps employed in practicing the method of the present inventioncomprise: producing a plastic tube in a softened condition; introducingthe softened tube into a die having a bore with a helical groovetherein, applying a pressure differential across the wall of the tube inthe die while rotating the die relative to the tube to urge the wall ofsaid tube into the helical groove of the die to form a helicalcorrugation in the tube wall, frictionally engaging the softened tubebefore entering the die to prevent rotation of the tube with said die,cooling the tube as it progresses through the die to set the helicalcorrugations in the tube, and removing the tube from the die.

Preferably, the tube is drawn through the die at a linear ratecorresponding to the formula: V=W/N where Vis the linear velocity of thetube in feet/minute, W is the angular velocity of the rotating tube diein revolutions/minute, and N is the number of helical groove revolutionsper linear foot measured axially in advantageous for forming plastictubing wherein the plastic is a thermoplastic fluorocarbon, such as acopolymer of fluorinated ethylene and fluorinated propylene (FEP),tetrafluoroethylene (TFE), and the like. In the case of PEP, thematerial is softened by heating in the tube-producing step and extrudedat about 650 F. and is cooled in the cooling step to about 60 F. n theother hand, in the case of TFE, the material is not extruded but apre-formed tube of TFE is softened by heating to about 680 F., passedthrough the die, and then cooled to about 60 F. In each case a pressuredifferential across the tube wall in the die of about pounds per squareinch should be maintained to obtain optimum results.

For the purpose of practicing the foregoing method, and in accordancewith the primary object of the present invention, apparatus is providedfor making continuous lengths of helically corrugated plastic tubing.Referring now to the drawings, FIG. 1 illustrates one embodiment of atube-fanning apparatus comprising an extruder which may be anyconventional extruder for thermoplastic material, and a cooling orquenching chamber shown generally at 11. The extruder 10 has a dischargeend 100 in which is provided an annular orifice 12 through which anextrudate 13 is expressed. A plurality of cartridge-type electrical.

heaters 14,14 are mounted in the extruder 10 adjacent the discharge end100 to heat the extrudate 13 into a softened flowable condition. Ifdesired, other heaters of the band-type may be provided around theperiphery of the extruder to provide additional heating capacity.

In the instant description, the extrudate 13, is described as being acopolymer of fluorinated ethylene and fluorinated propylene (FE?) andthe heaters 14,14 soften the FEP by heating it to a temperature of about650 F. The heated extrudate 13 is drawn into the form of a hollow conehaving a substantially smooth outer peripheral surface as it leaves thedischarge end of the extruder. In the present instance, the outersurface of the hot, conically shaped extrudate is subjected toatmospheric ambient conditions; however, if desired, a funnel-shapedmember may be provided adjacent the extruder to provide additionalsupport and guidance for the extrudate.

The exact amount of draw-down which may be imparted to the extrudatedepends upon a number of variables such as: the properties of thematerial being extruded, the temperature at which the extrusion takesplace, the distance between the extruder and the die, and many otherfactors which would be apparent to one skilled in the art. The draw-downmay be expressed in terms of a ratio between the diameter of theextrudate as it leaves the extruder and its diameter within the die. Inthe present invention, a draw-down ratio of at least 5 to l ispermissible; however, optimum results occur with a draw-down ratio ofbetween 3 to l and 2 to 1. In addition, a cone angle of from 60 to 90 isparticularly desirable, the cone angle being the apex angle of theconical extrudate.

After the extrudate 13 is produced in a softened condition, it must behardened to set the wall into a desired configuration and to impartstrength to the resultant tubing. For this purpose, the cooling chamber1 1 is provided adjacent the discharge end 10a of the extruder 10. Thecooling chamber 11 has a front wall 15 and a rear wall 16 and is adaptedto be filled with a cooling fluid to the level indicated at A; in thepresent instance the cooling fluid is water. The cooling chamber 11 ishermetically sealed by a door or lid 17 which is secured to the frontand rear walls 15 and 16, respectively, by threaded connections showngenerally at 18 and 19, respectively.

The flow of cooling water to the cooling chamber 1 l is controlled bymeans of a valve 20, the valve 20 being adjusted to provide a flow ratesufficient to maintain a temperature in the chamber which is adequate toproperly set the extrudate, which in the present invention is about 60 FThe water level in the chamber 11 is controlled by an overflow 21 whichtelescopes into an upright overflow pipe 22, and a conventional clamp 23is provided on the pipe 22 to lock the overflow 21 in any preselectedposition. A pump 44 is provided below the cooling chamber and isconnected to the overflow pipe 22 to draw the cooling water from thecooling chamber, and as more fully explained hereinafter, the pump 44also reduces the pressure in the cooling chamber 11. In addition, adrain valve 24 is provided in the bottom of the chamber 11 to permit thecooling fluid to be drained from the chamber when the die is beingchanged. Furthermore, it may be desirable in some instances to operatewith a fluid level which is below the extruded tubing in the coolingchamber, in which case the cooling fluid may be sprayed onto the surfaceof the tubing by a suitable spraying apparatus.

For the purpose of shaping the extrudate preparatory to entering thecooling chamber, a die 25 is provided intermediate the extruder and thecooling chamber. The die 25 (FIGS. 1 and 2) has an axial bore 26 inwhich is formed a helical groove 27, the die 25 being coaxially alignedwith the extruder 10 below the water level A, so that the cooling waterflows into the die to cool the hot extrudate and to set the wallthereof. As may be readily seen in FIG. 2, the die has a grooved portion28 remote from the extruder 10 which spirals radially outward from theaxis of rotation of the die and which extends axially in the directionof the cooling chamber 11. This radially spiraling portion of the diereduces the friction between the extrudate and the die and promotes thepositive disengagement of the extrudate from the groove to therebyobviate any tendency which may be present for the corrugations to jamwithin the groove.

In order to facilitate the rapid removal and replacement of the die, thedie is mounted on the front wall 15 of the cooling chamber 11 on a plate30 which has a hub 33 projecting forwardly and a sleeve 31 projectingrearwardly into the cooling chamber 11 through an aperture 32 in thefront cooling chamber. A drive wheel 35 is mounted on the hub 33 by ahearing assembly 36, and the drive wheel 35 has a centrally recessedportion 37 facing the extruder 10 which is internally threaded toreceive mating threads on the periphery of the die 25. A seal 38 is alsomounted in the front face of the hub 33 to engage the die 25 and toprevent air from leaking into the cooling chamber. Therefore, when thedie is threaded onto the drive wheel, the rearward portion 28 thereofrotates in the sleeve 31, so that with this mounting arrangement, thedie 25 may be rapidly removed from the drive wheel 35 with a minimumamount of tools and skill. Moreover, the die 25 may then be replacedwith another die having a different bore and groove characteristics tothereby provide a helical tube-forming apparatus which may be quicklychanged to accommodate various size tubing.

For the purpose of positively forcing the extrudate into the helicalgroove in the die, means is provided for applying a pressuredifierential across the wall of the hot extrudate when the extrudate iswithin the die. To this end, a vent 43 is provided in the extruderbetween the discharge end 100 and the exterior of the extruder tothereby provide fluid communication between the interior of the hollowconical extrudate l3 and a body of fluid at a reference pressure, whichin the present instance is air at atmospheric pressure. A pres suredifferential is produced across the tubing well in the die 25 by avacuum pump 44 which is connected to the cooling chamber 11 through theupstanding overflow pipe 22. The groove 27 in the die 25 provides fluidcommunication between the peripheral surface of the extrudate in the dieand the interior of the cooling chamber 1 1, so that when the pressureis reduced in the cooling chamber 11, the pressure is similarly reducedin the groove 27 in the die. Therefore, with atmospheric pressure in theinterior of the extrudate 13 and with a predetermined vacuum or negativepressure in the groove 27, the extrudate 13 is forced into the grooveand adopts substantially the configuration shown in FIG. 2. In order toregulate the vacuum in the cooling chamber 11, a valved vent 41 isprovided above the water level in the chamber so that by properlyadjusting the valve, atmospheric air may be bled into the coolingchamber 11. In the present instance an adjusted pressure differential ofabout 5 pounds per square inch across the wall of the extrudate 13 issufficient to force the extrudate 13 into the groove; however, otherpressures may also produce adequate results depending upon thetemperature and properties of the extrudate. If desired, the pressuredifferential may be adjusted to vary the depth of the corrugations inthe tubing, and with an appropriate control of the vacuum pump 44 or thevalved inlet 41, the pressure difi'erential may be periodically variedto produce tubing having different corrugations in spaced intervalstherealong. Furthermore, the depth of the corrugations may be varied insuch a manner to provide intervals in which there are no corrugationspresent in the tubing merely by properly controlling the vacuum pump andthe valved inlet.

In order to support the tubing in the cooling chamber while thecorrugations are being set, and for the purpose of providing a sealedopening through which the tubing passes out of the cooling chamber, aflexible closure member 39 is mounted on the rear wall 16 of the chamber11. The closure member has a cylindrical opening 39a axially alignedwith the die to thereby support the tubing substantially horizontallywithin the cooling chamber. In addition, a ring 40 and bolts 42 areprovided to sealingly secure the closure member 39 to the coolingchamber 1 1.

As noted heretofore, the die is threadedly secured to a drive wheelwhich is rotatably mounted on a hub on the cooling chamber. In order toimpart helical corrugations to the extrudate, the die is rotatedrelative to the extrudate, and, for this purpose, means is provided forrotating the die. The rotating means comprises a conventional belt andpulley drive shown generally at 45, the belt being of the V-type andengaging a V- shaped groove in the periphery of the drive wheel 35. Apulley 46 engages the belt 47, and the pulley 46 is connected to avariable-speed drive motor shown schematically at 48. Therefore, whenthe drive motor 48 is energized, the belt and pulley drive arrangementrotates the drive wheel 35 and the die 25 around the extrudate 13 as itis advanced to corrugate the extrudate. In addition, tubing may beproduced having intervals in which no corrugations are present merely byperiodically arresting the rotation of the die.

For this purpose of smoothly forming the helical corrugation in theextrudate, means is provided for drawing the extrudate through the dieat a velocity which is synchronized with the rotation of the die. Tothis end, a pair of take-up belts 50 and 51 are provided to apply thenecessary tension to the extrudate 13 to draw it through the die. Thetake-up belts 50 and 51 are driven at a predetermined speed which issynchronized with the rotation of the die 25, so that the angularvelocity of the die may be adjusted to correspond with the pitch of thehelical groove in the die to form tubing having a desired pitch ornumber of helices per unit of length. In addition, the take-up belts 50and 51 are aligned with the die 25 and are spaced apart a predetermineddistance to engage the wall of the extrudate 13 and to compress it intoa substantially, oval shape. In this manner, the take-up belts not onlyprovide the tension required to draw the extrudate through the die, butthey also cooperate to limit the tendency of the extrudate to rotatewith the die while the convolutions are being formed therein.

As noted above, the rotation of the die tends to impart an axial twistto the extrudate between the extruder and the die when the helicalcorrugations are being formed. If the twist is excessive, the wall ofthe extrudate kinks and permits air to enter the die, whereupon thevacuum in the die is lost or at least reduced to the point wherecorrugations are no longer formed in the tubing. Moreover, the tendencyto rotate with the die is also undesirable because it adversely affectsthe formation of the helical corrugations. Although the take-up beltscooperate with the closure member in the rear wall to reduce thesetendencies; nevertheless, they must not squeeze so tightly as topermanently deform the tube. Therefore, in accordance with the presentinvention, additional means is provided to limit the rotation of theextrudate with the die.

The rotation-limiting means comprises a stationary anti-rotation andsealing plate 55 mounted intermediate the cooling chamber 11 and theextruder 10, the anti-rotation and sealing plate 55 being bolted to theplate 30 so that the die 25 and plate 55 may thereby be rapidly removedand replaced. The plate 55 has a circular aperture 56 coaxially alignedwith the bore in the die, and a bevelled surface 57 around the aperture56 confronts the extruder 10 to guide the extrudate 13 into the die. Aseal 58 is provided between the die 25 and the anti-rotation plate 57 tothereby define a cavity or recess 59 around the extrudate 13 between theaperture 56 and the entrance to the die 25. Passage means, including thehelical groove 27 in the die 25, provides fluid communication betweenthe cavity 59 and the interior of the cooling chamber 11, so that when avacuum is applied within the cooling chamber, the vacuum is also appliedwithin the groove 27 and the cavity 59. With the interior of theextrudate 13 at atmospheric pressure, and the exterior thereof at thesubatmospheric pressure in the cavity 59, the extrudate 13 is urged intofrictional engagement with the aperture 56. The anti-rotation platethereby prevents the extrudate from twisting excessively and kinking sothat the vacuum in the die is preserved. Moreover, the intimate contactbetween the extrudate and the aperture provides an effective sealagainst the entry of air into the die. Thus, with this arrangement, theanti-rotation plate 55 cooperates with the take-up belts 50 and 51 andthe closure member 39 to greatly reduce the rotational tendency of theextrudate while simultaneously preserving the vacuum in the coolingchamber.

For the purpose of further improving the pressure distribution in thedie and the cavity, a modified die -125 (FIG. 3) may be provided havinga manifold 160 with a plurality of laterals 161 which open into a groove127. In this manner, parallel passages are provided in the die 125 tothereby more evenly distribute the pressure within the groove and thecavity and to improve the circulation of cooling fluid in the die.

In operation, the extruder 10 is started and the extrudate 13 is fedinto the die 25 after passing through the aperture in the anti-rotationplate 55. The vacuum pump 44 is also energized to apply a vacuum in thegroove in the die, and the vent valve 41 is adjusted to regulate thevacuum in the cooling chamber. A controlled flow of cooling water issupplied to the cooling chamber 11, the flow rate being adjusted by thevalve to maintain the proper cooling temperature in the chamber. Whenthe motor 48 is energized, the belt and pulley drive arrangement rotatesthe die on its axis and around the extrudate l3, and the take-up belts50 and 51 are displaced at a predetermined linear velocity to draw theextrudate through the die. The vacuum in the cooling chamber 11 isapplied to the helical groove 27 in the die 25 to produce a pressuredifferential across the wall of the extrudate and to force the extrudateinto the groove. At the same time, the cooling chamber vacuum is alsoapplied in the cavity 59 surrounding the extrudate 13 at the entrance tothe die, so that the extrudate frictionally engages the aperture 56 inthe anti-rotation plate to thereby sealingly grip the extrudate andreduce its tendency to rotate with the die. The take-up belts 50 and 51also grip the formed tube and cooperate with the anti-rotation plate 55to further reduce the rotational tendency of the extrudate. In thismanner, a continuous length of seamless, helically corrugated plastictubing is produced in an uninterrupted process. Furthermore, theapparatus of the present invention is particularly suited for beingquickly changed to produce tubing having various sizes and shapes ofhelical corrugations with a minimum amount of tools and with a minimuminterruption in production.

Although the described method and apparatus is particularly suited forproducing helically corrugated tubing from copolymers of fluorinatedethylene and fluorinated propylene, by extrusion; nevertheless, theinstant invention may also be employed to produce helically corrugatedtubing from any melt-extrudable thermoplastic material, for example,polyethylene, polypropylene, vinyl polymers and copolymers,chlorotrifluoroethylene polymers, nylon and the like.

As previously stated the method and apparatus of the present inventionare equally suited for producing helically corrugated tubing frommaterials such as tetrafluoroethylene (TFE) which are notmelt-extrudable. This is effected by heating a length of pre-formedtubing into a softened condition and feeding the softened TFE tubinginto a rotating die in such the same manner as the melt-extrudablematerial. The principal difference between the processing ofmelt-extrudable and non-melt extrudable material is that the extruder isreplaced with means for heating pre-formed tubing into a pliablecondition before it enters the rotating die.

In the case of TFE tubing, a preformed TFE tube of selected uniformdiameter and wall thickness is produced by the conventional pasteextrusion process in which the TFE material is first extruded and thensintered. To produce corrugated TFE tubing according to the presentinvention, a continuous length of the preformed TFE tubing is advancedaxially through a heater in which the tubing is heated to a temperatureabove the gel point to soften the tube material sufficiently to enablethe helical corrugations to be formed therein by a rotating die. Beforeentering the rotating die, and to prevent the softened tube from beingtwisted and kinked by the rotating die, the softened tube passes throughanti-rotation means which frictionally engages the tubecircumferentially thereof to restrain the tube from rotating with thedie as the helical corrugations are formed therein. The corrugated tubeis then cooled in a cooling chamber to permanently set the corrugationstherein.

Referring to the drawings, FIG. 4 illustrates an embodiment of apparatusfor producing helical corrugated TF E tubing which comprises generally aprimary heater 200, anti-rotation means 205, a rotating die 210 and acooling chamber 230. The heater 200 has a cylindrical chamber 201extending therethrough, and the diameter of the chamber 201 issubstantially greater than the outside diameter of the pre-formed TFEtubing being processed in order to provide space between the tubing andthe surrounding wall of the chamber to eliminate or minimize linear dragor tension on the tubing as it moves through the heater. During passageof the tubing through the heater 200 the tubing is heated to atemperature above the gel point (about 621 F.) of the TFE material, forexample, to about 650 F. in order to soften the TFE materialsufficiently to enable the helical corrugations to be fonned therein. Inthe present instance this is accomplished by radiant heat supplied, forexample, by electric heating coils 202 provided with suitable controls(not shown) to insure heating of the TFE tubing to the desiredtemperature.

As previously described, the rotation of the die tends to impart anaxial twist to the softened TFE tubing with the result that the softenedtubing becomes kinked and distorted unless the softened tubing isrestrained against rotation with the die. Accordingly, upon leaving theheater 200 and before entering the rotating corrugating die 210, theheated softened TFE tubing passes through an anti-rotation device 205having means for frictionally engaging the softened tubecircumferentially thereof with sufficient braking or restraining forceto prevent the tube from rotating with the corrugating die so that thesoftened tubing will not be twisted or kinked before entering the die.

As shown in the drawings, the anti-rotation device 205 is of elongatedconfiguration having a cylindrical chamber 206 extending therethrough incoaxial alignment with the chamber 201 of the heater 200. For most ofits length the chamber 206 has a diameter slightly larger than theoutside diameter of the softened TFE tubing in order to minimize lineardrag on the tubing as it passes through the chamber 201. However,adjacent the outlet end of the device 205 the diameter of the chamber206 is reduced for a short distance inwardly from the end as shown toprovide a restriction or throat 207 having a diameter slightly less thanthe outside diameter of the heated softened TFE tubing. The transitionfrom the larger diameter portion of the chamber 206 to the restrictionor throat 207 is gradual as indicated by the slope or ramp 208 which isdisposed, for example, at an angle about 30 with respect to the axis ofthe chamber 206. The anti-rotation device 205 is provided with heatingmeans such as electric coils 209 surrounding the chamber 206 which serveto maintain the tubing at the desired temperature above the gel pointduring passage of the tubing therethrough.

The softened TF E tubing is pulled through the heater 200 andanti-rotation device 205 by the rotating die 210 as the latter forms thehelical convolutions in the softened tubing. As the softened tubingmoves through the restriction or throat 207 it is drawn and reducedslightly in diameter so that the restriction or throat 207 engages thesurface of the tubing circumferentially thereof with sufficient frictionforce to restrain the tube against rotating with the die and therebyprevent kinking and twisting of the tube that might otherwise result.

The rotating die 210 has axially projecting hub portion 21 1 rotatablyreceived within a bore 212 extending coaxially through the central hubportion 213 of a mounting plate 214 which is fixedly secured to theadjacent wall 231 of the cooling chamber 230. The die 210 and its hubportion 21 1 have an axial bore 215 extending therethrough and the wallof the bore 215 is formed with a helical groove 216 which, as the die210 is rotated, operates to form a continuous helical corrugation in thewall of the softened TFE tubing as the latter passes through the die.The die 210 is rotatably mounted on the hub 213 of the plate 214 by abearing 217 and is rotationally driven at the desired speed by a belt218 which passes about a pulley 219 provided peripherally of the die210. The belt 218 in turn is driven by a second pulley 220 mounted on ashaft 221 and driven by a motor M. Suitable seals 222 and 223 areprovided, respectively, between the anti-rotation device 205 and therotating die 210 and between the latter and the mounting plate 214.

As in the case of the first embodiment of the invention, means isprovided for forcing the softened TFE tubing into the convolutions ofthe helical groove 216 in the bore 215 of the die 210. To this end theinterior of the softened TFE tubing is open to the atmosphere and apressure differential is produced across the tubing wall in the die 210by means of a vacuum pump (not shown). The vacuum pump is connected by apipe 224 to a passage 225 in the mounting plate 214 which in turnconnects to an annular manifold 226 in said plate 214 whichcircumscribes the bore 215 of the die 210. Communication between themanifold 226 and the helical groove 216 is provided by a plurality ofaxial slots or grooves 227 equally spaced circumferentially of the boreof the die and intersecting the radial apices of the helical groove 216as more clearly shown in FIG. 6. Therefore, with the interior of thesoftened TFE tubing at atmospheric pressure and a predetermined vacuumor negative pressure in the helical groove 216 the softened TFE tubingis forced into the groove 216 and is formed to the configuration shownin FIG. 5.

From the rotating die 210 the helically corrugated TFE tubing passesthrough the cooling chamber 230 wherein the tubing is cooled topermanently set the corrugated configuration formed by the rotating die210 into the wall of the tubing. For this purpose the cooling chamber230 is substantially filled with a cooling liquid, such as water, to thelevel indicated at A. The cooling water is supplied to the chamber 230by a pipe 232 controlled by means of a valve 233 which is adjusted toprovide a flow rate sufficient to maintain the cooling temperature inthe chamber 230 required to properly set the corrugated tubing, forexample, about 60F. The water level in the chamber 230 is controlled byan overflow 234 which telescopes into an upright overflow pipe 235provided with a clamp 236 to lock the overflow 234 in any preselectedvertical position. In addition, a drain 233 having a valve 239 isprovided at the bottom of the chamber 230 to permit the cooling liquidto be drained from the chamber.

To provide a seal at the opening through which the corrugated TFE tubingenters the cooling chamber 230 and also to assist in supporting the TFEtubing in the chamber 230 while being set, an annular flexible closuremember 240 is mounted on the adjacent inner wall of the mounting plate214 by means of a ring 241 and bolts 242. The diameter of the openingthrough the seal 240 is such that the inner peripheral edge portion ofthe seal 240 engages within the helical configurations formed in the TFEtubing as best shown in FIG. 5. In addition, and for the same purpose, asimilar flexible annular closure member 243 is provided at the outletopening in the opposite wall 244 of the cooling chamber 230 throughwhich the corrugated TF E tubing exits from the cooling chamber. As inthe case of the seal 240, the seal 243 may be secured in place by a ring245 and bolts 2%.

The corrugated TFE tubing is drawn through the die 210 and coolingchamber 230 at a velocity synchronized with rotation of said die by apair of opposed take-up belts 250 and 251 mounted respectively on rolls252 and 253. The rolls 252 and 253 are driven by the motor M at apredetermined speed to correspond with the pitch of the helical groove216 in the die 210 to form TFE tubing having the desired pitch or numberof corrugations per unit of length. The belts 250 and 251 are spacedapart a predetermined distance to engage the surface of the TFE tubingwith sufficient pressure not only to provide the tension or pullrequired to draw the TFE tubing through the die 210 but also tocooperate with the restriction or throat 207 in preventing rotation ofthe tubing with the die 210 as previously described. However, the belts250 and 251 must not squeeze the tubing so tightly as to permanentlydeform the tubing. As in the case of the first embodiment, the TFEtubing preferably is drawn through the die 210 at a linear ratecorresponding to the aforesaid formula V=W/N within the ranges of speedof rotation and pitch previously described.

In practicing this embodiment of the invention, the preformed TFE tubingis passed through the heater 200 where it is heated above the gel pointof the TFE material to about 650 F. The softened TFE tubing then passesthrough the anti-rotation device 205 and into the rotating die 210.Vacuum is applied to the groove 216 in the die 210 and is adjusted toprovide the desired pressure differential across the wall of the tubing. A controlled flow of cooling water is supplied to the coolingchamber 230, the flow rate being adjusted by the valve 233 to maintainthe proper cooling temperature in the chamber. When the motor M isenergized, the belt and pulley drive arrangement rotates the die 210 onits axis and around the heated softened TFE tubing, and the take-upbelts 250 and 251 are driven at a predetermined linear velocity to drawthe softened tubing through the die. Just prior to entering the rotatingdie 210 the TFE tubing is drawn through the restriction or throat 207and is frictionally engaged thereby circumferentially with sufficientforce to restrain the tubing from rotating with the die 210. The take-upbelts 250 and 251 also cooperate with the throat 207 to further reduceany rotational tendency of the tubing. In this manner, a continuouslength of seamless, helically corrugated TFE tubing is produced in anuninterrupted process. Furthermore, the apparatus of the presentembodiment is particularly suited for quickly changing the die 210 toproduce tubing having various sizes and shapes of helical corrugations.

' Apart from the material previously described, other materials, forexample, materials which are strengthened by vulcanization may also beprocessed by the method and apparatus of the present invention. Forinstance, such materials may be formed in their thermoplastic state andset by vulcanization with suitable heating equipment rather than beingset with the cooling chamber employed in the apparatus of the presentinvention.

While certain preferred embodiments of the present invention have beenillustrated and described, it is in tended that changes andmodifications may be made without departing from the invention asdefined in the appended claims.

lclaim:

l. The method of making helically corrugated tetrafluoroethylene tubingwhich comprises axially advancing a continuous length of preformedcylindrical tetrafluoroethylene tubing, heating said advancing restrainifo ce, to revent rotation f said softened tubing, iii rociucmg he drawnand circumferentially restrained softened tubing while still heatedaxially into a rotating die having a bore provided with a helicalgroove, the predetermined circumferential restraining force exerted onthe softened tubing being sufficient to hold said tubing againstrotation with the die and thereby prevent the softened tubing from beingtwisted and kinked by said die before entering the die, applying adifferential pressure across the wall of said softened tubing in saiddie while rotating said die relative to said tubing to urge the softenedtubing wall into said helical groove to form a helical corrugation insaid tubing wall, cooling the corrugated tubing to permanently set saidhelical corrugation in said tubing wall, and engaging and drawing thecooled corrugated tubing axially away from the die at a predeterminedlinear speed to cause the softened tubing to traverse the die at avelocity synchronized with the speed of rotation of the die.

2. Apparatus for making corrugated tetrafluoroethylene tubing comprisingmeans for axially advancing a continuous length of preformed cylindricaltetrafluoroethylene tubing, means for heating the advancing preformedtubing to a temperature above the gel point thereof to soften the tubematerial, means for drawing the softened tubing to a smaller diameterand simultaneously frictionally engaging the softened tubingcircumferentially thereof with a predetermined restraining force toprevent rotation of said softened tubing, a die rotatable about its axisand having an axial bore provided with a helical groove for receivingthe heated softened tubing, means for rotating the die at apredetermined speed, the predetermined circumferential restraining forceexerted onthe softened tubing by said drawing means being sufficient torestrain the tubing from rotating'with the die and prevent the rotatingdie from twisting and kinking the softened tubing before entering thedie, means operable to produce a pressure differential across the wallof said tubing in said rotating die to urge said wall into the helicalgroove to form a helical corrugation in the tubing wall, meanssynchronized with said die rotating means for drawing said tubular bodyaxially through said rotating die to form a helical corrugation in saidtubing wall, cooling means containing a cooling medium operable to coolsaid tubular body and set said helical corrugation in said tubing wall,and means engaging and drawing the cooled corrugated tubing axially awayfrom the die at a predetermined linear speed to cause the softenedtubing to traverse the die at a velocity synchronized with the speed ofrotation of the die.

UNITED STATES PATENT DFFMIE CERTIFICATE OF Patent No. 3,692,889 DatedSeptember 19, 1972 Inventor(s) Arthur R. Hetrich It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the Abstract, line 3, "disc" should read die Col. 4, line 59, after"chamber" insert wall Col. 8, line 16, "such" should read much Signedand sealed this 8th day of May 1973.

(SEAL) Attest:

EDWARD M.FLETCHER-,JR. v ROBERT GOTTSCHALK Attesting OfficerCommissioner of Patents FORM PO-IOSO (10-69) USCOMM-DC 50376-P69 a us.GOVERNMENT PRINTING OFFICE: 1969 O-366334

2. Apparatus for making corrugated tetrafluoroethylene tubing comprisingmeans for axially advancing a continuous length of preformed cylindricaltetrafluoroethylene tubing, means for heating the advancing preformedtubing to a temperature above the gel point thereof to soften the tubematerial, means for drawing the softened tubing to a smaller diameterand simultaneously frictionally engaging the softened tubingcircumferentially thereof with a predetermined restraining force toprevent rotation of said softened tubing, a die rotatable about its axisand having an axial bore provided with a helical groove for receivingthe heated softened tubing, means for rotating the die at apredetermined speed, the predetermined circumferential restraining forceexerted on the softened tubing by said drawing means being sufficient torestrain the tubing from rotating with the die and prevent the rotatingdie from twisting and kinking the softened tubing before entering thedie, means operable to produce a pressure differential across the wallof said tubing in said rotating die to urge said wall into the helicalgroove to form a helical corrugation in the tubing wall, meanssynchronized with said die rotating means for drawing said tubular bodyaxially through said rotating die to form a helical corrugation in saidtubing wall, cooling means containing a cooling medium operable to coolsaid tubular body and set said helical corrugation in said tubing wall,and means engaging and drawing the cooled corrugated tubing axially awayfrom the die at a predetermined linear speed to cause the softenedtubing to traverse the die at a velocity synchronized with the speed ofrotation of the die.